July 16, 2011

Where do Eukaryotes come from? The distinguishing feature of eukaryotes is the stately nucleus that adorns every eukaryotic cell. The best theory to explain how nuclei came to be is called the endosymbiotic theory. The origin of the earliest eukaryotic cells was one of the leading mysteries in biologist, but the researcher Lynn Margulis offered a compelling solution to this enigma with her endosymbiosis theory. The theory can adequately explain many mysteries about the origin of the Eukaryotic domain. While much the process remains elusive, scientists believe they have a fairly accurate conception of how the first endosymbiosis occurred. Some archean bacteria have a rigid cell wall that keeps its shape, but at some point, this cell wall disappears in a certain archean lineage. Through either a faulty cell wall gene or a complete excision of the genes that controls cell wall production, a cell’s insides were no longer boxed in by a cell wall and the only thing keeping the contents of our cell from leaking into the outside world is a tenuous cell membrane. This underlying cell membrane is fluid and allows cells’ edges to fold in on themselves, creating bubbles called vacuoles. Rather than sucking in dissolved chemicals through the cell wall, this archean cell can engulf cell fragments and even whole smaller cells by folding itself over the cell to be eaten, and then pinching itself off to form a vacuole around it. This process is called phagocytosis and it engulfs macromolecules in vacuoles.

An archaean cell like the one described above could flop around without a cell wall and fold over its food to eat. One of these archaean cells swallowed a living a bacterium that managed to survive in the cytoplasm of the cell that ate it. This bacterium somehow was kept from being digested by a lysosome within the host cell and lived long enough to replicate with the host cell time and time again. Eventually, this bacteria found a nice little home within the archaean cell, living sheltered from the dramatic changes in the environment that the archaean has to face. Scientists tend to think that the unlucky bacterium that got engulfed was a rickettsia bacteria; this is important because rickettsia can detoxify peroxides into water. Peroxide is a free radical inside cells, hacking apart the cellular machinery and the rickettsia is useful for the archaean cell because it takes a poisonous chemical and turns it into water. This process repeats itself on Areios, creating the first eukaryotic cells by endosymbiosis.

Because the engulfed cell relied on the host for maintaining homeostasis, mutations or deletions in its genome for certain biological pathways could get destroyed without impacting the cell’s ability to reproduce. For instance, lodged inside another cell meant that genes responsible for locomotion could erode without hurting the viability of the engulfed bacteria. Eventually, the engulfed cell lost much of its cellular mechanics and diminished in size until it was no smaller than an organelle. Yet unlike other organelles, like mitochondria and chloroplasts, which were created by this process of endosymbiosis differ in two unique ways. First of all, they house DNA inside of a cell yet outside of the nucleus where DNA is ordinarily kept. And two, they have a membrane that shuts them off from the rest of the cell that is wholly different from the cell membrane that separates the cell from the outside world.

Endosymbiosis has occurred several times in the history of eukaryotic cells as seen in euglenas and other protists that have more than one membrane surrounding their organelles. This is evidence that a proto-euglena swallowed an alga, and a secondary symbiosis had occurred in that family. This would give the euglena’s mitochondria two membranes surrounding it; the original membrane that surrounded the mitochondria of the alga, and the second cell membrane came from the alga itself that went through the same process described above, where its genome and machinery could get whittled down until just the cell membrane remained.

The earliest eukaryotes could have arisen from a symbiotic relationship with an engulfed aerobic heterotrophic prokaryote

July 11, 2011

Check out this link to Astrobiology Magazine about a three-day excursion on the Atacama desert in Chile, considered to be the driest desert on Earth. This continuing 3-part series called Islands of Life highlights Field Research Editor Henry Bortman’s trip with researchers studying Mars and its potential habitability for life. Because the Atacama Desert’s climate resembles that of early Mars, studying life in this barren region may give scientists clues about what kind of life could have survived Mars’ early environment.

July 5, 2011

ATP synthase is an enzyme embedded at the end of the electron transport chain that creates ATP. Protons from outside the cell pass through the ATP synthase enzyme into the cell. This energy drives the ATP synthase to string phosphates onto a molecule of adenosine and create ATP. Areiosan cells function remarkably in the same way, except ATP synthase strings molecules of arsenate to a molecule of mercapto-adenosine, which is an analogue of adenosine that features sulfur built into the structure. It’s remarkable how analogous our biochemistry is with Areiosan life. Despite a different set of chemicals, the reactions inside our cells seem to mirror those on Areios. Our biochemistry is so analogous that it is reasonably clear to suggest that life adheres to a specific set of chemical pathways and even from one world to another, the same kinds of chemical reactions are preserved, albeit with slight changes in the chemical reagents used.

All life uses an electron transport chain to shuttle electrons through their cell membrane, powering pumps to make their fuel source, the molecule called adenosine troposphere (ATP). The electron transport chain for an organisms that can undergo photosynthesis begins when a discrete packet of light called a photon gets passed around through a cell‘s machinery. First of all, light can behave as a particle called a photon. Photons can come in different ‘colors’ that correspond to the wavelength of that photon. The visible light spectrum ranges from blue-violet on one end of the spectrum, and red on the other end of the spectrum. Blue light has the shortest wavelength of visible light while red light has the longest wavelength. Wavelength corresponds with how many times a wave of light cycles from start to finish. Each wave can be thought of a single photon, so blue light has more energy per unit length because a shorter wavelength means more waves per unit length and therefore more energy and photons are available.

Photosynthetic organisms on Earth rely on mainly red and yellow light to power photosynthesis, but this need not be the case. Scientists proffer that plants could use light as far from the lower end of the infrared spectrum to the upper end of the ultraviolet spectrum. The output of the parent star determines the color that a plant will utilize; red and yellow light are the most abundant wavelength of photon emitted from our Sun, so the vast majority of organisms utilize that most abundant source of energy rather than blue or green, which is not as available. But Areios’ star Hemera is characteristically dimmer than our Sun, so it shines with an orange-red glow. For algae on Areios, they tend to absorb more green and yellow and reflect blue and violet light. Because there is no plant life on Areios, redish and purple algae are among the few photosynthesizing organisms on the planet and the ocean surfaces are covered in it, giving much of the world a blood red or violet hue. These algae are amongst the oldest photosynthetic life forms on Areios, and while they cannot undergo photosynthesis or produce oxygen, they play a major role in most ocean ecosystems; because they can tolerate high concentrations of salts in Areios’ briny seas, they are the basis for several aquatic food webs. Some of these purple algae don’t rely on chlorophyll at all, but use a pigment similar to rhodopsin, like the coloring found in human retinas, to absorb light and power their ATP synthesis.
But every photosynthetic creature faces a limitation on how far down the visible spectrum they can utilize. Called the red edge, plants on Earth avoid absorbing light coming from the infrared end of the spectrum because they have to protect themselves from overheating. This isn’t as big of a problem for Areioan life because Areios is on the whole colder than Earth in terms of average temperatures.

Infrared red light is essentially what we perceive as heat and while algae on Areios can absorb infrared light, they too meet a limit on much they tolerate. Their red edge is around infrared wavelengths of 1.5 μm or so, but there are other organisms on Areios that can tolerate much greater into the infrared. Some creatures can see well into the infrared rage enough that their eyesight does not depend on any visible light. Their eyesight would be nearly identical to the infrared telescopes that are used to study astronomical bodies. Perhaps the most astonishing impact of this is that some creatures on Areios would be able to see the universe from a totally different perspective than humans; their infrared vision could view the oldest objects in the universe unaided by a telescope or observatory like we humans must use. Perhaps most amazing of all is that these creatures can do so without the need for cryogenic coolants. Our earth-borne telescopes need to be cooled down to near-absolute temperatures to work in the far infrared spectrum, but Areiosans are not encumbered by that limitation at all. Outside their murky atmosphere lies an exotic universe that they can see with their own eyes. Or eye.

This photo of the milky way galaxy shows a different view of our galaxy when viewed in the infrared. This is how our universe looks from view of Areiosan life.